With the development of wireless multimedia services, demands of people for high data rate and user experience are increasing, so as to present higher requirements for system capacity and coverage of a traditional cellular network. In addition, the popularization of applications such as social networking, near field data sharing and local advertising enable demands of people, for knowing people or things of interest nearby and communicating therewith, to gradually increase. A conventional cell-based cellular network has obvious limitations in high data rate and supporting of proximity services. Under this demand background, a Device-to-Device (D2D) technology, which is representative of a new development direction of future communication technologies, emerges. Application of the D2D technology can alleviate burdens on the cellular network, reduce the power consumption of a battery of a User Equipment (UE), increase the data rate, improve the robustness of network infrastructure, and well meet requirements for the foregoing high data rate services and proximity services.
The D2D technology can work at a licensed band or an unlicensed band to allow multiple UEs (i.e., D2D UEs) supporting D2D functions to perform direct discovery/direct communication in the presence of network infrastructure or in the absence of network infrastructure. There are mainly three D2D application scenarios as follows.
1) UE1 and UE2 perform data interaction under the coverage of a cellular network, and user plane data does not pass through network infrastructure, as shown in Mode 1 of FIG. 1.
2) UE in a weakly covering/coverless area performs relayed transmission, as shown in Mode 2 of FIG. 1. UE4 with poor signal quality is allowed to communicate with a network via proximal UE3 covered by the network, and coverage expansion and capacity improvement of an operator can be assisted.
3) When an earthquake or an emergency occurs and a cellular network cannot normally work, inter-device direct communication is allowed, as shown in Mode 3 of FIG. 1. Control planes and user planes among UE5, UE6 and UE7 perform one-hop or multi-hop data communication without network infrastructure. The D2D technology usually includes a D2D discovery technology and a D2D communication technology.
1) The D2D discovery technology refers to a technology for judging/determining whether two or more D2D UEs are proximal to each other (e.g., within a range capable of performing D2D direct communication) or judging/determining whether a first UE is proximal to a second UE.
2) The D2D communication technology refers to a technology for direct communication of some or all communication data between D2D UEs without network infrastructure.
In application scenarios as shown in FIG. 2 and FIG. 3, a D2D UE may serve as a relay node, such that a remote D2D UE at the coverage edge of a cellular network or out of coverage can perform cellular communication with a network through a relay node UE, and the D2D UE performs D2D communication through the relay node UE. The D2D UE probably moves at the coverage edge of a cellular cell, and in this case, the D2D UE needs to change an access node thereof for accessing the network. For example, after moving into the cell coverage area, the D2D UE relayed by the relay UE can access the network through an evolved Node B (eNB), and for a UE within the cell coverage area which accesses the network through the eNB, the UE can access the network only through the relay UE after moving out of the cell coverage area. A method for changing an access node of a D2D UE is not provided in the conventional art, thus influencing the continuity of communication services of UEs at the coverage edge of a cell.